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Hum Measurements
I've recently rebuilt the circuit board of my Champ clone that I built
a number of months for a number of reasons the biggest being an objectionable amount of hum. 60 Hz was plainly audible plus along with a much louder 120 Hz buzz. Now I am very happy with the results of the modifications both in terms of greatly reduced hum and improved tone. However, there is still a tiny bit of audible hum. I know it is not coming from the pre amp since its level doesn't change with the volume setting and because it is still there when I remove the 12AX7. On the scope I see a small signal at the speaker terminals. Removing the speaker and connecting a 10 Ohm resistor I observed a signal of about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component both of which are audible. Further measurement revealed some faint higher frequency noise of about 7 mV p-p. From these measurements I conclude that in the worst case the power delivered to the 10 Ohm load is less than 1.5 uW! The resulting hum is very low in my opinion as I don't recall ever hearing a quieter guitar tube amp. Nevertheless, on my next project I will endeavor to reduce it further. I was just surprised that such a small signal would be audible at all. Is that in line with other builders experience? Joe |
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